This proposal is motivated by the recent report of the Institute of Medicine's Committee on HIV Prevention Strategy in the United States, which advocates the use of evaluation and cost-effectiveness frameworks for resource allocation decisions. This project aims to promote reasoned planning and policy making in the realm of HIV and substance abuse prevention via the development of mathematical and economic models. To achieve this goal, the investigators propose the following specific aims: 1. To construct production functions that characterize the relationship between expenditures to modify risk behaviors and empirically observed changes in those behaviors. 2. To develop model-based methods of translating the behavioral impact of HIV-related interventions into epidemiologically meaningful outcome measures. 3. To assess how best to estimate health outcomes, economic outcomes, and cost-effectiveness of HIV intervention programs. 4. To assess the effectiveness and cost-effectiveness of specific interventions aimed at slowing the spread of HIV and drug abuse, including substance abuse treatment programs, strategies to reduce the spread of resistant HIV, and HIV counseling, testing, and referral. 5. To guide prevention strategy and the allocation of societal resources by examining the effectiveness and cost-effectiveness of portfolios of interventions. This includes assessing the appropriateness of epidemiologically targeted versus general HIV prevention programs, as well as analyzing other national and international HIV prevention strategies. 6. To examine the relationship between policy modeling and public health decision making in the areas of HIV and substance abuse. The proposed research will advance the state-of-the art in HIV planning and policy modeling, critically evaluate promising interventions, and help provide the scientific foundation for allocation of scarce resources to portfolios of HIV and substance abuse prevention interventions.